Green roofs

Growing plants on top of buildings can be a good thing

A green roof demonstration project at the Chicago Center for Green Technology

PHOTO BY ANDREW CISCEL / COURTESY OF FLICKR

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How can a residential or commercial roof be modified
to provide environmental benefits?

Most buildings are designed to shed rain and so are
built with hard, impenetrable roofing surfaces. As a result, rainwater
bounces off and collects as runoff, picking up impurities — including
infectious bacteria from animal waste, as well as harmful pesticides and
fertilizers — on the way to municipal storm sewers, which in turn
eventually empty out into local bodies of water.

Minimizing this run-off means that more impurities
will remain in local soils, where they can be broken down more easily into
their constituent elements than if they are concentrated downstream. To
achieve this goal, landscape architects have developed so-called green
roofs, which involve the placement of living plant matter and soil on top
of a building absorb, collect, and reuse rainwater while preventing
run-off. There are abundant uses for the water green roofs collect, from
watering exterior plantings at ground level to flushing toilets inside.

According to Steven Peck of the Toronto-based
nonprofit Green Roofs for Healthy Cities, green roofs can play an important
role in maintaining ecological integrity within otherwise paved-over areas.
"The roofscapes of our cities are the last urban frontier —
from 15 percent to 35 percent of the total land area — and the green
roof industry can turn these wasted spaces into a force for cleaner air,
cleaner water, energy savings, cooling, beauty, and recreation," he
says.

The U.S. Environmental Protection Agency encourages
the creation of green roofs to mitigate the urban "heat island
effect," whereby temperatures in crowded cities can soar some 10
degrees Fahrenheit higher than in less developed areas nearby. Other
benefits, says the EPA, include providing amenity space for tenants (in
effect, replacing a yard or patio), reducing building heating and cooling
costs through the buffering effect of the plant matter and soil, filtering
pollutants such as carbon dioxide out from air and heavy metals from
rainwater, and increasing bird habitat in otherwise built-up areas.

In addition to going all out to build a
"living" green roof, builders can use certain inorganic
materials to make an existing roof greener. The nonprofit Cool Roof Rating
Council, for instance, suggests roofing surfaces that reflect the
sun's heat so as to reduce the urban heat-island effect while
improving residential energy efficiency. According to the group, "a
cool roof reflects and emits the sun's heat back to the sky."
Builders can check out the CRRC's Web site for a database of
information on the radiative properties of various roofing surfaces to help
them make the smartest choices for their clients and for the environment.

Another quality that makes certain roofs greener than
others is how long they last. Metal roofs are known to be relatively
maintenance free and last longer than shingles in most situations. Slate
roofs also have an excellent reputation for long life, although getting
work done on them can be expensive when they do need repairs. The Slate
Roofing Contractors Association reports that sea-green slates can last
anywhere from one to two centuries, depending on where the slate is
quarried and how well it's eventually installed.